The broad objective is to study the physiology of the mammalian kidney, in particular that of the renal medulla. Our investigations center upon three general topics: urinary concentrating mechanisms, microcirculation of the renal medulla and functional nephron heterogeneity. Micropuncture and realted microanalyses and television microscopy are used to study normal rodents and two special strains of rats--one with hereditary diabetes insipidus (Brattleboro) and the other with a long renal papilla (Munich-Wistar). Urinary concentration mechanism: 1. Micropuncture of the Psammomys in hydropenia and volume expansion; 2. Microperfusion of the Psammonys decending and ascending thin limb; 3. Stationary microperfusion of the Psammomys descending and ascending thin limb; 4. Intracortical location of the glomeruli supplying the long loop of Henle in the papilla; 5. The effect of various maneuvers of the (TF/P)C1/ (TF/P)Na ratio at the end of the descending limb. Microcirculation of the renal medulla: 1. To continue the determination of red cell velocity in vasa recta capillaries in several conditions: normal hydropenia, water diuresis, and ADH-induced antidiuresis. Nephron heterogeneity: Potassium secretion by the juxtamedullary nephron: 1. Effect of amiloride on potassium secretion in the juxtamedullary nephron in chronic potassium loaded rats. BIBLIOGRAPHIC REFERENCES: Johnston, Paul A., Carlos A. Battilana, Frank B. Lacy and Rex L. Jamison. Evidence for a concentration gradient favoring outward movement of sodium from the thin loop of Henle. J. Clin. Invest. 59: 234-240, 1977. Battilana, Carlos A., Jahar Bhattacharya, Frank B. Lacy and Rex L. Jamison. Effect of chronic potassium loading on potassium secretion by the descending limb pars recta of the juxtamedullary nephron. Clin. Res. 25: 165A, 1977. (abstract)